Here, I posted this in another thread here. This is how I feed my corns and how I gauge when to move up in prey size:

Quote Originally Posted by blueapplepaste
Here's my fail safe method of feeding a corn that pretty much works in all situations. I feed all of mine once a week, whether they are hatchlings or adults, once a week (the exception is some of the larger ones are every 10-14 days), but 90% get fed once a week.

Feed something that is 1.5x's the widest part of your snakes body. Feed it this size once a week until you no longer see a lump after it eats. Once you no longer see a lump, move up in prey size. Keep this up until you stop at large mice. Even if there is no lump, keep feeding this size. I used to move up to small rats and stop there, but I found that my snakes started becoming pudgy and more prone to be lethargic, I found stopping at large mice to be the ideal point.

So if yours is on fuzzies, feed him one fuzzy a week until you no longer see a lump. Then move to hoppers until you no longer see a lump, then to small mice, etc.

This is the way I do it and the way I tell others who ask to do it. There are other methods, but I like this one because you don't need a scale or aren't worrying about if its been 5 days or 6 days or whatever. Just pick a day of the week and then that day becomes feeding day and its easy to remember.